1. Field of the Invention
The present invention relates to an ink jet printing head and an electronic machine incorporating an ink jet printing head such as, for example, a printer, a word processor, a facsimile machine or a plotter.
2. Description of the Related Art
Ink jet printing heads of a Kyser system are disclosed in U.S. Pat. Nos. 4,216,483, 4,189,734 and 4,158,847. As shown in FIG. 15, a printing head of a Kyser system is generally composed of: a multiplicity of individual ink passages 104, each including a supply passage 101, a pressure chamber 102 and an end portion 103, provided on a photosensitive glass substrate 100 at regular intervals; a diaphragm 110 (omitted in FIG. 15, see FIG. 16) attached to the glass substrate 100 in such a manner as to cover all the individual ink passages 104; and piezoelectric elements 120 attached to the diaphragm 110 at the positions corresponding to the respective pressure chambers 102 of the individual ink passages 104. The individual ink passages 104, the diaphragm 110 and the piezoelectric elements 120 are generally provided on both sides of the glass substrate 100 in order to enhance the degree of integrity as shown in FIG. 16, and the printing head is disposed in an inclined manner with respect to the direction of printing.
In such a printing head, an electric field is applied to the piezoelectric element 120 so as to displace it, whereby the corresponding portion of the diaphragm 110 is moved, thereby forcing ink out of the end portion 103 of the corresponding individual ink passage 104.
In this type of printing head, a glass substrate which allows isotropic etching is generally used, and the individual ink passages are formed on the glass substrate by isotropic etching. However, it is difficult to control the etching depth in isotropic etching, so that in order to increase the depth of the individual ink passage (the dimension of the individual ink passage perpendicular to the direction of extension and to the direction of array), it is necessary to increase the width of the individual ink passage (the dimension of the individual ink in the direction of array). On the other hand, in order to increase the density of the printing head, it is necessary to reduce the width of the individual ink passage. With due consideration of these conflicting requirements, in this type of conventional printing head, the depth of the individual ink passage is increased as much as possible while realizing a certain degree of high density.
Nevertheless, since isotropic etching is adopted, it is inevitable that the depth of etching (the depth of the individual ink passage) is insufficient. Especially, in the case of forming the pattern of the individual ink passages such as that shown in FIG. 2 in order to enhance the density, the narrower the width of the individual ink passage 104 is made, the smaller the depth thereof becomes, as shown in FIG. 13, which is the sectional view of the individual ink passage shown in FIG. 2, taken along the line B--B. If the depth of the individual ink passage becomes small, the resistance of the passage against the flow of ink increases, which leads to various problems. For example, it is impossible to jet ink, it is necessary to raise the driving voltage for the piezoelectric element, or it is impossible to increase the driving frequency for the piezoelectric element.
In isotropic etching, since the individual ink passage is inevitably shallow, the cross section of the end portion 103 of the individual ink passage 104 is a semicircle, as shown in FIG. 14. This causes inconveniences such as nonuniformity in the shapes of ink droplets, satellite phenomenon (ink dribbles after it is jetted) or instability in the direction of ink jetting, thereby lowering the printing quality.